The usefulness of rigid foamed polymeric boards in a variety of applications is well known. For instance, polymeric foam boards are widely used as isulating structural members.
In the past, infrared attenuating agents (IAAs) such as carbon black powdered amorphous carbon, graphite, and titanium dioxide have been used as fillers in polymeric foam boards to minimize material thermal coductivity which, in turn, will maximize insulating capability (increase R-value) for a given thickness. Thermal conductivity, k is defined as the ratio of the heat flow per unit cross-sectional to the temperature drop per unit thickness with the US unit:
            Btu      ·      in              Hr      ·              Ft        2            ·              °F        .                  And    ⁢                  ⁢    the    ⁢                  ⁢    metric    ⁢                  ⁢    unit    ⁢          :        ⁢                  ⁢          W              m        ·                  °K          .                    The heat transfer through an insulating material can occur through solid conductivity, gas conductivity, radiation, and convection. The total thermal resistance (R-value), R is the measure of the resistance to heat transfer, and is determined as:R=t/k Where, t thickness.
Rigid foamed plastic boards are extensively used as thermal insulating materials for many applications. It is highly desirable to improve the thermal conductivity without increasing the density, and/or the thickness of foam product. Particularly, the architectural community desires a foam board having a thermal resistance value of R=10, with a thickness of less than 1.8″, for cavity wall construction, to keep at least 1″ of the cavity gap clean.
It is also desirable to improve the UV stability, particularly for such as exterior wall insulation finishing system (EIFS), and highway and railway underground applications where prolonged exposure of sun light of the surface of the polymer foam boards are usually occurred in job-sites.
It is also desirable to improve the dimensional stability at elevated temperature and/or high humility for such as indoor pool roofing, exterior wall insulation. U.S. Pat. No. 5,679,718 illustrates a microcellular extruded polystyrene foam containing graphite as an infrared attenunating agent (IAA). The IAA provide a greater proportional reduction in foam thermal conductivity than foams having a larger cell size.
U.S. Pat. No. 6,420,442 shows a flame-proofed polystyrene foam material containing expanded graphite having a particle size of from 20 to 1000 micrometers. The foam is flame-retardant.
U.S. Pat. No. 6,213,540 illustrates an extruded thermoplastic foam having a high compressive strength. The thermal resistance of the foam may be enhanced by adding fillers such as carbon black particulates, clay particulates or carbon or graphite fibers.
U.S. Pat. No. 3,574,644 shows a method of increasing the flame retardance of normally flammable materials by adding expandable graphite. Expanded graphite having a particle size of from about 10 to 325 mesh may be added to a film forming agent or other polymeric materials such as polyesterds, polystyrene and polyethylene. Larger particles of graphite are preferred because the expansion ration of the larger flake is greater than that of a smaller flake.
U.S. Pat. No. 5,719,199 shows a flame retardant flexible, polymeric foam which contains expandable graphite as a fire retardant.
U.S. Pat. No. 5,854,295 illustrates a microcellular, polymeric foam containing an infrared attenuating (IAA) agent to reduce thermal conductivity. The IAA may be particulate flakes of metals or carbonaceous substances such as carbon black and natural or synthetic graphite.
U.S. Pat. No. 6,387,968 shows a method for producing water expandable styrene polymers using water as a sole blowing agent. Styrene is polymerized in an aqueous suspension in the presence of from 0.1 to 15% by weight of a solid substance insoluble in water and in styrene such as carbon black or graphite. The graphite has an average particle size from 2 to 20 micrometers.
U.S. Patent Application Publication No. 2001/0036970 shows polymer foam having improved thermal insulating performance. The thermoplastic foam, typical made from polyethylene rein or polypropylene resin or blends thereof, may contain an IAA such as carbon black, graphite, or titanium dioxide to enhance thermal insulating capability.
U.S. Patent Application Publication No. 2003/0082343 illustrates a bendable thermoplastic foam which contains a slit to reinforce the foam. The foam may contain additives such as thermally insulating additives including aluminum, gold, silver, carbon black and graphite.
European Patent Application No. 729,000 shows a fire-retardant polymer composition containing 100 parts by weight polyolefin or polystyrene, 1-30 parts by weight of an oxide or a complex oxide of metals such as antimony, boron and moybdenum and 1-30 parts of heat-expandable graphite. The graphite of a preferred particle size distribution contains the particles passing through a 80-mesh sieve at a content of 20% or lower by weight, more preferably from 1% to 20% by weight. Graphite containing the particles at a content of lower than 1% by weight would slightly impair the shape-retaining properties of the resin composition when the resin composition is exposed to fire.
All of the above patents teach foams with 1 to 30% by weight graphite—either natural or synthetic graphite, or expandable graphite in the particle size around 2 to 1000 micrometers, having decreased thermal conductivity, and improved flame resistance.
Regular low density foams have very thin cell wall thickness in the range of 0.2 to 6 microns. Particularly, in order to enhance the insulation R-value, a target cell wall thickness of less than 0.1 micron is needed.
Thus, there is a need to graphite having at least in one dimension—usually the thickness of the plate shaped graphite in nano-scale, i.e., less than 0.1 microns or 100 nanometers. It is an object of the present invention to provide a process for preparing low density extruded polymer foams containing multi-layered nano-graphite which has good processing properties, and improved foam physical properties, including thermal conductivity, ultraviolet (UV) radiation resistance, dimensional stability, mechanical strength, flame spread rate and smoke density.